PHP＆Mysql - 两个表之间的左外连接

Question

我有两个名为'events'和'topics'的表，每个表可以有很多注释。

我需要做的是列出所有事件和主题与每行的评论量。我设法返回所有主题，这很好，但我不知道如何将事件表添加到MySql。下面列出了评论和事件表字段。任何人都可以用这个查询来帮我吗？

活动：

• ID
• EVENT_NAME

评论：

• POST_ID < - 对于任何一个事件或主题表

• 表中的相关型号ID < - 该行属于这样无论是主题或事件

  SELECT 
      t.id, t.title, c.created_at, 
      IF(ISNULL(c.allComments), 0, c.allComments) AS totalComments 
  FROM topics AS t 
  LEFT OUTER JOIN (
      SELECT created_at, post_id, COUNT(*) AS allComments 
      FROM comments 
      GROUP BY post_id 
  ) AS c ON c.post_id = t.id 
  ORDER BY tc.created_at DESC, c.allComments DESC 
  

Answer 1

听起来事件和主题应该是同一个表。

不过，我想我们可以用UNION来做到这一点。活动和主题有我希望的相同专栏？ （或至少是同样重要的？）

(SELECT c.table as event_or_topic, e.*, count(C.table), MAX(C.created_at) as latest_c 
FROM events E LEFT JOIN comments C on (C.post_id = E.id) 
WHERE C.table = 'Events' 
GROUP BY C.post_id) 
UNION 
(SELECT c.table as event_or_topic, t.id*, count(C.table), MAX(C.created_at) as latest_c 
FROM topics T LEFT JOIN comments C on (C.post_id = E.id) 
WHERE C.table = 'Topics' 
GROUP BY C.post_id) 
ORDER BY latest_c 

请注意，ORDER BY适用于整个UNION，而不是单独的SELECT。

使用LEFT JOIN应该允许没有评论的那些行仍然显示。我认为问题在于我们的部分选择依赖于评论（即--Ctable，在最后的评论中排序等）。计数应该没问题 - 如果没有任何评论，只会为零。

您可能需要稍微更改SELECT部分​​。我想显示C.table，以便知道某行是否是主题或事件，但恐怕可能会使计数错误。除了伯爵之外，你还需要评论吗？您在查询中使用了post_id和table之外的其他列，而您忽略了在您的问题中进行解释。

你还有列，我不知道它们是什么，就像评论的zoneTable

Answer 2

试试这个表：

SELECT t.id, t.title, c.created_at, COUNT(c.allComments) AS totalComments FROM topics AS t LEFT JOIN comments c ON t.id=c.post_id GROUP BY t.id ORDER BY tc.created_at DESC, c.allComments DESC

Answer 3

如果我理解你的问题，你有3个表：

-Events

-Topics

- 评论

如果这是真的这样的事情应该提取所有的数据：

SELECT * 
FROM events,topics 
LEFT JOIN comments ON post_ID = ID 
ORDER BY date DESC 

希望我沿着正确的路线！

W.

Answer 4

我有它的工作。如果任何人这样做的更好的和有效的方式知道，那么请让我知道：

(SELECT t.id, t.title, tc.dateCreated AS commentDate, 
IF(ISNULL(tc.allComments), 0, tc.allComments) AS totalComments, 
t.LastActive as dateChanged 
    FROM Events AS t 
    LEFT OUTER JOIN (
      SELECT MAX(created_at) AS dateCreated, post_id, 
      COUNT(*) AS allComments 
      FROM comments 
      GROUP BY post_id 
     ) AS tc ON tc.post_id = t.id) 
UNION 
(SELECT t.id, t.title, tc.dateCreated AS commentDate, 
IF(ISNULL(tc.allComments), 0, tc.allComments) AS totalComments, 
t.LastActive as dateChanged 
    FROM topics AS t 
    LEFT OUTER JOIN (
      SELECT MAX(created_at) AS dateCreated, post_id, 
      COUNT(*) AS allComments 
      FROM comments 
      GROUP BY post_id 
     ) AS tc ON tc.post_id = t.id) 
ORDER BY commentDate DESC, dateChanged DESC, totalComments DESC